An organic EL device is a spontaneously luminous device which features higher brightness and higher legibility than those of the liquid crystal devices enabling vivid display to be attained and has, therefore, been vigorously studied.
In 1987, C. W. Tang et al. of the Eastman Kodak Co. have developed a device of a layer-laminated structure comprising various kinds of materials to bear individual roles, and have put an organic EL device using organic materials into a practical use. The above organic EL device is constituted by laminating a fluorescent body capable of transporting electrons and an organic material capable of transporting holes. Upon injecting both electric charges into the layer of the fluorescent body to emit light, the device is capable of attaining a brightness of as high as 1000 cd/m2 or more with a voltage of not higher than 10 V.
So far, very many improvements have been made to put the organic EL device to practical use. For example, the organic EL device has been widely known having a structure comprising an anode, a hole injection layer, a hole-transporting layer, a luminous layer, an electron-transporting layer, an electron injection layer and a cathode which are arranged in this order on a substrate more finely dividing their roles than ever before. The device of this kind is achieving a high efficiency and a high durability.
To further improve the luminous efficiency, attempts have been made to utilize triplet excitons and study has been forwarded to utilize a phosphorescent luminous compound.
In the organic EL device, the electric charges injected from the two electrodes recombine together in the luminous layer to emit light. Here, to improve the luminous efficiency, to lower the driving voltage and to lengthen the life, it is necessary that the device has excellent carrier balance enabling the electrons and holes to be efficiently injected and transported, and enabling them to be efficiently recombined together.
As the hole injection material used for the organic EL device, there were, first, proposed phthalocyanines such as copper phthalocyanine (CuPc) (e.g., see a patent document 1) accompanied, however, by an absorption in the visible band. Therefore, materials having a phenylenediamine structure have now been widely used (see a patent document 2).
As the hole-transporting material, on the other hand, arylamine materials having a benzidine skeleton have heretofore been used (see a patent document 3).
Tris(8-hydroxyquinoline) aluminum (Alq3) which is a representative luminous material has been generally used as the electron-transporting material. However, the electron mobility possessed by the Alq3 is lower than the hole mobility possessed by the hole-transporting material that is generally used. Besides, the work function of the Alq3 is 5.8 eV which cannot be said to be a sufficiently large hole blocking power. Therefore, use of the above hole-transporting material is accompanied by a problem in that the holes partly pass through the luminous layer to deteriorate the efficiency.
In order to efficiently inject the holes or the electrons from the anode and cathode into the luminous layer, further, there has been developed a device obtained by laminating the hole injection layers and the electron injection layers each in a number of two or more layers to set stepwise the ionization potential values and the values of electron affinity possessed by the materials (see a patent document 4). With the materials that are used, however, none of the luminous efficiency, driving voltage or device life is still satisfactory.
At present, in order to improve properties of the organic EL devices, attempts have been made to attain a high efficiency, a low driving voltage and a long life maintaining good carrier balance by using in combination materials that excel in hole and electron injection/transport property and in maintaining stability and durability in the form of thin films.